In the field of magnetic random access memories (MRAMs), a “spin torque transfer switching technique” for causing magnetization switching by applying a current to a magnetic material is well known.
By the spin torque transfer switching technique, a spin-injection current as a write current is applied to a magnetoresistive element, and spin-polarized electrons generated there are used to cause magnetization switching. Specifically, the angular momentum of the spin-polarized electrons is transmitted to the electrons in the magnetic material serving as the magnetic recording layer, so that the magnetization direction of the magnetic recording layer is reversed.
By using the spin torque transfer switching technique, local magnetization states can be readily controlled on a nanoscale. Further, the value of the spin-injection current can be made smaller with miniaturization of magnetic materials. This facilitates the realization of spin electronics devices such as high-recording-density hard disk drives and high-recording-density magnetic random access memories.
For example, a magnetic random access memory includes a magnetoresistive element as a memory element having a magnetic tunnel junction (MTJ) film that utilizes a tunneling magnetoresistive (TMR) effect. The MTJ film is formed with three film films: a recording layer and a reference layer made of magnetic materials, and a tunnel barrier layer interposed between the recording layer and the reference layer. The MTJ film stores information depending on the magnetization states of the recording layer and the reference layer. In a spin-injection MRAM using the spin torque transfer switching technique, information writing into a magnetoresistive element is performed by applying a current in a direction perpendicular to the film plane of the MTJ film.
Known magnetic layers that can be used in magnetoresistive elements include perpendicular magnetization films each having a magnetization direction in a direction perpendicular to the film plane, and in-plane magnetization films each having a magnetization direction in the in-plane direction. Where perpendicular magnetization films are used, the leak field generated by the magnetization of the reference layer is directed perpendicularly to the film plane of the recording layer, and therefore, a magnetic field having a large perpendicular component is applied to the recording layer. The leak current that is generated from the reference layer and is applied to the recording layer normally acts in such a direction that the magnetization of the recording layer is made parallel to the magnetization of the reference layer. If the recording layer is larger than the reference layer, however, the leak current generated from the reference layer is unevenly applied to the recording layer, and the spin torque transfer switching characteristics are degraded. Therefore, the size of the recording layer needs to be made as small as or smaller than the size of the reference layer.
By using the spin torque transfer switching technique, highly-integrated MRAMs can be realized. In doing so, however, the magnetoresistive elements of memory cells need to be made as small as several tens of nanometers in size. It is normally difficult to perform dry etching on materials that are used in magnetoresistive elements and contain magnetic metals such as Co and Fe. Therefore, the sidewalls cannot be formed almost vertically to achieve high integration. By forming the recording layer on the lower side (the substrate side), the magnetic characteristics of a magnetoresistive element using perpendicular magnetization films can be more improved. That is, if the sidewalls formed by performing conventional dry etching in a magnetoresistive element have small angles, the size of the recording layer becomes larger than the size of the reference layer, and therefore, the spin torque transfer switching characteristics are degraded as described above.
Further, each magnetoresistive element has a stacked structure in which a recording layer and a reference layer are stacked, with a thin tunnel barrier layer being interposed in between. In this stacked structure, the distance between the recording layer and the reference layer is short. When a magnetoresistive element is formed by processing such a stacked structure, the recording layer and the reference layer containing magnetic metals are trimmed as well as the tunnel barrier layer. As a result, the metals might redeposit to the entire sidewalls of the stacked structure across the tunnel barrier layer. In such a case, another leak current path is formed due to the redeposition, and the recording layer and the reference layer are short-circuited. As a result, defective magnetoresistive elements are formed, and the magnetoresistive element yield becomes lower.